1863 - Cairo IL Received Cancel

Here's a wild guess,  caveat,    I know nothing about US covers.

That cover was just 5 months after the battle of Corinth,   perhaps receival of mail was jeopardised by damage to infrastructure of postal facilities,
the transit mark may have been required as the last point of established postal networks, before delivery to Corinth.
Some mail earlier had been transitted by Military courier to Corinth.
We don't see any Corinth markings on covers around that date.
Something to consider.

I saw your extensive posting on SCF,  if the cover had indeed travelled by steam boat, then any mail disembarking may have been required to receive a transit strike.
I recall similar requirements on mail leaving Port Tewfik, on the Suez.



 

I am changing my guess

The Railroad rolled into Cairo in 1856
I reckon it may be an Rail Station or RPO transit mark.


Reference to Army Riverboats of the period

17-3. “I will accept no terms but unconditional surrender.” view image Uncommon design, “Maj. Gen. Grant, The Hero of Fort Donelson,” with somewhat primitive woodcut of a rather tired Grant, in field cap. Smaller printed caption, “Gen. Grant’s reply to Buckner on the terms of surrender: ‘I will accept no terms but unconditional surrender....’” Manuscript postal marking, “...Feby. 28.” To “John W. Wertenberger, Helena, Arkansas, 76 [Ohio] Regt., Co. K, in Care of Capt. J.M. Jay, to follow the Regt.” Bold black backstamp, “Received Cairo Ill. / Mar. 6, 1863....” Cairo was a key Union base under Grant’s command. Strategically located at the junction of the Ohio and Mississippi Rivers, it was vital to controlling river traffic. From Cairo, Grant launched his campaigns to gain control of the entire Mississippi, from Fort Donelson to Vicksburg. The envelope was probably carried from Cairo to Helena, Ark. on an Army riverboat. Old pencil notations on verso, “Scott $25.00” and “1928,” the former probably the old New York dealer whose name still graces the standard philatelic catalogues today. Two hinge traces, generally uniform toning, few stains, else very good. $275-375

That elevates you to the cutting edge of the discipline, when information is not available or sparse.
This is the transit line between "collector" and "philatelist" if you continue the study and investigation.
Good Luck
The wiki  image of Cairo in 1885 illustrates how busy that confluence was, and how it probably got its name.